The gaseous mixture produced in the regenerator of fluid catalytic cracking (FCC) process systems contains solid particulates, including catalyst fines, and acidic gases, such as sulfur oxides. It is desirable to lower the level of both solid particulates and acidic gases from such a gaseous mixture before it is introduced into the atmosphere in order to minimize the release of pollutants to the atmosphere. Government regulations require that the concentration of sulfur dioxide from a FCC process unit be 25 ppmv (dry basis at 0% O2) or less in order for the FCC process unit to qualify for clean unit status.
It is known that solid particulates can be removed from gases by wet gas scrubbing methods including scrubbing gases in venturi scrubbers in which the scrubbing liquid, and particulate-containing gases are mixed under pressure. The gases or vapors are passed with the scrubbing liquid through a constricted passage of the scrubber wherein intimate mixing of scrubbing liquid and gas occurs. Generally, the effluent of the scrubber (which may be one or more venturi structures in series or in parallel) is passed to a separator in which the contaminated liquid is separated from the cleaned gas. It is also known that acidic or basic materials may be added to the scrubbing liquid to neutralize or absorb basic or acidic contaminants that may be present in the gas that is subjected to the wet scrubbing treatment.
In order to remove an acidic gas, such as SO2, from a gaseous effluent stream wet gas scrubbing will typically use a basic material, preferably a caustic. The caustic reacts with the SO2 and removes it from the effluent stream. A significant amount of work has been done over the years to improve wet gas scrubbing technology. For example, U.S. Pat. Nos. 3,970,740 and 4,600,567 teach a wet gas scrubbing process for the removal of catalyst fines and acidic gases from the FCC regenerator gaseous effluent by scrubbing with an aqueous basic solution. The basic solution is maintained within a critical pH range of about 6 to 7 in a jet ejector venturi scrubbing system. The basic material is selected from alkali hydroxide, ammonia or ammonium hydroxide.
U.S. Pat. No. 6,139,807 teaches an apparatus for removing SO2 from flue gas streams wherein the flue gas stream is contacted in a scrubbing zone with an aqueous ammonium or ammonium hydroxide solution. It is also mentioned that a caustic, such as sodium or potassium hydroxide, can also be used. This '807 patent also requires an additional step which is the oxidation of the absorption liquid to produce ammonium sulfate that can be used in the production of fertilizer.
While various wet gas scrubbing processes have met with various degrees of technical and commercial success, there still remains a need in the art for improved wet gas scrubbing processes capable of economically achieving lower levels of acidic gases.